The present invention generally relates to a dual waveband lens, and in particular, to such lens which simultaneously focuses MMW and IR energy in a single focal plane.
A variety of lens systems have been developed for use in receiving and focusing electromagnetic energy. Most such lens systems are designed for use within a single region of the electromagnetic spectrum, such as the infrared or visible region.
Lenses find application in most optical devices; one of their purposes is to focus waves of electromagnetic radiation at a uniform focal point. Lenses achieve this by altering the direction of the electromagnetic wave as it passes through the lens medium. Lenses do not alter frequency, but the velocities of wave components are altered. Velocity within a lens is directly proportion al to wave length, consequently the effective index of refraction may be said to vary as a function of wavelength. Since the difference in the indices of refraction is small within a waveband region, the indices of refraction may be neglected in situations where the wavelengths are substantially similar. However, in situations where the wavelengths of interest are disparate, the optics will generally not perform similarly in both of the wavebands. Optics configured to focus radiation from a first waveband in a focal plane generally will not be suitable for simultaneously focusing radiation from a distant, second waveband, in the same focal plane.
Some lens systems have been developed for use with more than one region of the electromagnetic spectrum. Multiple band-region lens systems find application in numerous military and industrial detection systems. Such systems are useful because they allow the user to select the waveband that results in optimal detection, tracking and accuracy. In general, millimeter waves are more effective than infrared over long distances and in adverse weather conditions. For instance, the mm-wave waveband provides superior image data in inclement weather. The mm-wave readily penetrates rain, fog, other inclement weather, and even some opaque solids. Consequently the mm-wave based sensor may provide superior data in both long range, and environmental penetration applications. Conversely, the infrared waveband provides superior resolution but does have the range or penetrating power of the mm-wave waveband. Thus for optimal resolution and range a system capable of detecting in both wavebands is desirable.
Existing systems for creating dual wave-band lenses have relied on various means of separating the waveband components and then processing the individual components. Such systems include placement of two detectors, wherein one is placed at the first focal length and wherein the second is placed at the second focal length. These systems provide a combination system that is useful for both energy bands but necessarily obscures a portion of one of the wavebands. Such image obstruction is undesirable. Alternatively, some systems use a beam splitter to divide the incoming energy into two components. Each component is subjected to a different focusing system. Once focused the two bands may be projected onto a single focal plane. These devices usually use the same energy collection aperture and then direct the different types of energy to separate and different sensing devices using complex optical arrangements. This approach suffers from difficult problems in combining the data from the separate sensors, a process known as data fusion. Additionally the beam splitter can result in appreciable signal loss. Only a fraction of the incident energy from each band component ever reaches the detector. In many applications such losses may not be desirable or even acceptable. Additionally, the complexity of such a system often renders the system both expensive and unwieldy.
A desirable solution to these problems would comprise a system wherein a single lens could simultaneously focus two or more different energy band-regions at the same focal point.
The present invention provides an apparatus, wherein a single lens, or aggregation of lenses in a substantially columnar sequence, simultaneously focuses two or more different energy band-regions in substantially the same focal plane. Wherein said apparatus includes a first element that is exposed to the incident dual waveband radiation, the first element is comprised of a first composition and has a finite thickness, a perimeter, a first index of refraction, and a first and second surface substantially perpendicular to said perimeter. The first surface has a first geometry, or surface shape, and the second surface has a second geometry, or surface shape. The second element is comprised of a second composition and has a thickness, a perimeter, a second, and usually different, index of refraction, and a first and second surface substantially perpendicular to said perimeter. The first surface has a third geometry and the second surface has a fourth geometry. The first and second elements are aligned such that the second surface of the first element and the first surface of the second element are in substantial conformity with each other and aligned in such a way that the perimeters essentially form a column.
Another embodiment of the present invention provides a method for fabricating a multi-element lens configured to simultaneously focus a plurality of wavebands in an approximately identical focal plane. This method comprises the steps of selecting a number of materials, usually two, if air-gaps are not counted, having unique indices of refraction and known refractive behavior for the plurality of wavebands. Knowing the refractive behavior is important in determining the transmission properties of a material. The next step is to configure the materials in the form of an apparatus. The shape of the apparatus may be predicted using successive approximation techniques to generate aspheric lens surfaces for ideal focii in one of the wavebands, and then confirm the solutions by ray tracing with Snell""s law in both bands.